开篇

 这篇文章是用来讲解清楚TC（Transaction Coordinator：事务协调器）在处理TM发送过来的begin操作（事务开启操作）。

 核心逻辑包括GlobalSession对象的生成、GlobalSession的持久化以及XID生成。

TC begin 流程说明

• 1.创建GlobalSession对象，GlobalSession.createGlobalSession()。
• 2.添加周期监听器到GlobalSession当中，生命周期对象为DefaultSessionManager。
• 3.启动GlobalSession的周期监听器，添加GlobalSession对象到全局sessionMap对象。
• 4.启动GlobalSession的周期监听器，持久化GlobalSession对象。

TC begin 源码分析

public class DefaultCore implements Core {@Overridepublic String begin(String applicationId, String transactionServiceGroup, String name, int timeout) throws TransactionException {// 创建全局GlobalSession对象GlobalSession session = GlobalSession.createGlobalSession(applicationId, transactionServiceGroup, name, timeout);// 全局GlobalSession对象添加生命周期监听器SessionHolder.getRootSessionManager()session.addSessionLifecycleListener(SessionHolder.getRootSessionManager());// 启动全局Session对象GlobalSessionsession.begin();// 返回新生成的XID返回return XID.generateXID(session.getTransactionId());}
}

说明：

• GlobalSession.createGlobalSession()创建全局GlobalSession对象。
• session.addSessionLifecycleListener()给GlobalSession对象添加生命周期监听器。
• session.begin()方法通过生命周期监听器保存全局GlobalSession对象。
• sessionHolder.getRootSessionManager()返回DefaultSessionManager对象。
• XID.generateXID()创建XID值。

public class GlobalSession implements SessionLifecycle, SessionStorable {// 生命周期监听器的容器private ArrayList<SessionLifecycleListener> lifecycleListeners =  new ArrayList<>();public static GlobalSession createGlobalSession(String applicationId, String txServiceGroup, String txName, int timeout) {GlobalSession session = new GlobalSession(applicationId, txServiceGroup, txName, timeout);return session;}public GlobalSession(String applicationId, String transactionServiceGroup, String transactionName, int timeout) {// 生成transactionId对象。this.transactionId = UUIDGenerator.generateUUID();this.status = GlobalStatus.Begin;this.applicationId = applicationId;this.transactionServiceGroup = transactionServiceGroup;this.transactionName = transactionName;this.timeout = timeout;}// 添加生命周期监听器public void addSessionLifecycleListener(SessionLifecycleListener sessionLifecycleListener) {lifecycleListeners.add(sessionLifecycleListener);}public void begin() throws TransactionException {this.status = GlobalStatus.Begin;this.beginTime = System.currentTimeMillis();this.active = true;for (SessionLifecycleListener lifecycleListener : lifecycleListeners) {lifecycleListener.onBegin(this);}}
}// 生成TransactionId的类和方法
public class UUIDGenerator {private static AtomicLong UUID = new AtomicLong(1000);private static int UUID_INTERNAL = 200000000;public static long generateUUID() {long id = UUID.incrementAndGet();if (id > 2000000000) {synchronized (UUID) {if (UUID.get() >= id) {id -= 2000000000;UUID.set(id);}}}return id;}
}

说明：

• GlobalSession构造器内部通过UUIDGenerator.generateUUID()生成transactionId。
• addSessionLifecycleListener()方法添加生命周期监听器DefaultSessionManager。
• begin()方法调用生命周期监听器的onBegin()方法(lifecycleListener.onBegin)，实现GlobalSession的持久化。

public class SessionHolder {private static final String ROOT_SESSION_MANAGER_NAME = "root.data";private static final String ASYNC_COMMITTING_SESSION_MANAGER_NAME = "async.commit.data";private static final String RETRY_COMMITTING_SESSION_MANAGER_NAME = "retry.commit.data";private static final String RETRY_ROLLBACKING_SESSION_MANAGER_NAME = "retry.rollback.data";private static SessionManager ROOT_SESSION_MANAGER;private static SessionManager ASYNC_COMMITTING_SESSION_MANAGER;private static SessionManager RETRY_COMMITTING_SESSION_MANAGER;private static SessionManager RETRY_ROLLBACKING_SESSION_MANAGER;public static void init(String sessionStorePath) throws IOException {if (sessionStorePath == null) {ROOT_SESSION_MANAGER = new DefaultSessionManager(ROOT_SESSION_MANAGER_NAME);ASYNC_COMMITTING_SESSION_MANAGER = new DefaultSessionManager(ASYNC_COMMITTING_SESSION_MANAGER_NAME);RETRY_COMMITTING_SESSION_MANAGER = new DefaultSessionManager(RETRY_COMMITTING_SESSION_MANAGER_NAME);RETRY_ROLLBACKING_SESSION_MANAGER = new DefaultSessionManager(RETRY_ROLLBACKING_SESSION_MANAGER_NAME);} else {if (!sessionStorePath.endsWith("/")) {sessionStorePath = sessionStorePath + "/";}ROOT_SESSION_MANAGER = new FileBasedSessionManager(ROOT_SESSION_MANAGER_NAME, sessionStorePath);ASYNC_COMMITTING_SESSION_MANAGER = new DefaultSessionManager(ASYNC_COMMITTING_SESSION_MANAGER_NAME);RETRY_COMMITTING_SESSION_MANAGER = new DefaultSessionManager(RETRY_COMMITTING_SESSION_MANAGER_NAME);RETRY_ROLLBACKING_SESSION_MANAGER = new DefaultSessionManager(RETRY_ROLLBACKING_SESSION_MANAGER_NAME);}}public static final SessionManager getRootSessionManager() {if (ROOT_SESSION_MANAGER == null) {throw new ShouldNeverHappenException("SessionManager is NOT init!");}return ROOT_SESSION_MANAGER;}
}

说明：

• getRootSessionManager()返回DefaultSessionManager对象，实现生命周期接口。

public class DefaultSessionManager extends AbstractSessionManager {public DefaultSessionManager(String name) {super(name);transactionStoreManager = new TransactionStoreManager() {@Overridepublic boolean writeSession(LogOperation logOperation, SessionStorable session) {return false;}@Overridepublic void shutdown() {}@Overridepublic List<TransactionWriteStore> readWriteStoreFromFile(int readSize, boolean isHistory) {return null;}@Overridepublic boolean hasRemaining(boolean isHistory) {return false;}};}
}public abstract class AbstractSessionManager implements SessionManager, SessionLifecycleListener {protected static final Logger LOGGER = LoggerFactory.getLogger(AbstractSessionManager.class);protected Map<Long, GlobalSession> sessionMap = new ConcurrentHashMap<>();protected TransactionStoreManager transactionStoreManager;protected String name;public AbstractSessionManager(String name) {this.name = name;}@Overridepublic void addGlobalSession(GlobalSession session) throws TransactionException {if (LOGGER.isDebugEnabled()) {LOGGER.debug("MANAGER[" + name + "] SESSION[" + session + "] " + LogOperation.GLOBAL_ADD);}transactionStoreManager.writeSession(LogOperation.GLOBAL_ADD, session);sessionMap.put(session.getTransactionId(), session);}@Overridepublic void onBegin(GlobalSession globalSession) throws TransactionException {addGlobalSession(globalSession);}
}

说明：

• DefaultSessionManager是GlobalSession的生命周期管理器。
• DefaultSessionManager的父类AbstractSessionManager实现SessionLifecycleListener接口。
• DefaultSessionManager的调用父类AbstractSessionManager的onBegin()方法。
• onBegin()方法内部执行addGlobalSession()方法添加GlobalSession对象。
• addGlobalSession()方法执行transactionStoreManager.writeSession()执行持久化，自定义的TransactionStoreManager啥都不操作。
• transactionStoreManager是DefaultSessionManager内生成TransactionStoreManager对象。
• addGlobalSession()方法执行sessionMap.put()保存GlobalSession对象。